In oil recovery and storage operations, hydrocarbon gases are produced from oil condensate storage tanks through flash losses, working losses, standing losses, and breathing losses, where liquid hydrocarbons evaporate into gaseous form. Working losses occur when the liquids are agitated, i.e., when new liquids are pumped into the tanks; breathing losses are primarily the result of diurnal heating of the tanks; and flash losses occur as the result of a sudden pressure drop which occurs when liquid hydrocarbons move from a separator (i.e., at approximately 100-250 psig) to an oil condensate storage tank at much lower pressure (i.e., 0 to 2 psig). Flash losses account for a significant portion of total losses. Collectively, these losses of hydrocarbon gases are referred to herein interchangeably as “flash emissions,” “vent gas,” “flash gas,” “vapor” and combinations thereof.
Flash emissions include Volatile Organic Compounds (VOC) and therefore pose a hazard to air quality as they form ground level ozone when they react with NOx. Historically, such vent gases were vented through relief valves of the storage tanks into the atmosphere.
To protect the environment and the health of the public at large, the permissible levels for ground level ozone and, in turn, uncontrolled flash emissions from oil storage tanks are regulated by various regulatory agencies at federal and state levels. Methods of flash emissions control include flaring, where the flash emissions are burned off, and more advanced systems of vapor recovery that capture, compress, and inject these flash emissions into nearby gas gathering pipelines, such as described in U.S. Pat. No. 7,350,581.
However, traditional systems are typically either sized for average flow rate of flash emissions, which tends to overwhelm such traditional systems during the peak rate of flash emissions when liquids migrate to the tanks; or peak flow rate, which involves extremely high capital equipment costs where the equipment only runs for brief periods throughout day, with minimal usage of compression capacity and increased maintenance costs.
As these gaseous flash emissions are saturated and exist at or slightly above atmospheric pressure, the present inventor has recognized a need to remove condensed liquids from these flash emissions to prevent catastrophic failure of downstream compression equipment, as well as a need to address the intermittent nature of flash emission production in traditional natural gas production. The present inventor has also recognized the need, in some circumstances, for reducing the oxygen content present in recovered vapors.